Linkage type front suspensions were prominent in motorcycling's early days, and have long been recognized for some real advantages over the common telehydraulic fork. But, like forks, most linkage systems have a vital downfall: they are hung from a steering head which is a long way from the axle.
One answer to fork problems is the Hossac system found on a road racer in England. Although it has the same functions as any other system, the elements are combined differently, to place the suspension between the steering head and main frame. This method appeared as a pure parallelogram linkage on cycles over fifty years ago. Advantages include rigidity (the A-arm supports are triangulated and overhang ratio is low), reduced steering inertia (the upright weighs one third of a fork), progressive action (with the supports as linkage), and low friction (no sticky sliders). Also, there are no mechanical limits on either travel or lock. It is a commendable effort, but the original suffered from steering-suspension interaction even with small displacements.
The original design had an angle between the handlebar axis (set approximately vertical) and the upright axis (set at rake angle). Thus some component of the motion of the upright ends of the linkage is one up, one down relative to the plane containing the handlebar ends, as well as back and forth.
This out-of-plane displacement changes the effective length of any sort of linkage. During superimposed suspension motion, there is relative angular displacement of the handlebars and upright: bump steering. This can eliminated by mounting the handlebar axis parallel to the upright axis, an impossible condition because the axis cannot be positioned far enough forward to place the grips properly for the rider at this tilt. The supports are in the way. Moving the grips forward relative to their axis results in the rider pulling back and out rather than in; non-ergonomic. It's a marginal condition even with the vertical axis. However, add a bellcrank the axis of which is parallel to the upright, and link it to both upright and handlebars. This removes out-of-plane displacements from the suspended portion of the linkage. Angularity effects are now found only between the handlebars and the bellcrank where there is no relative suspension motion. Therefore there is no bump steer from any linkage component. However, the angularity still causes a change in effective length of the links on the handlebar side of the bellcrank, which shows up as a strain there. Now it is necessary to mount the handlebars as two halves, each semi-bar on its own hinge, to relieve this strain. It becomes a change in included semi-bar angle with increased steering motion. Each handgrip then is slightly "progressive": i.e. increments of motion one way move the upright more than increments the other way. (If you view the rider's arms as links on the bars of a fork, it will be seen that some progression occurs there also.) By choosing pivot offsets appropriately the effect was tailored to the make the semi-bar on the inside of the turn fold back significantly. Another facet of the opposed progression is that pulling back, as on uphills, quickens the steering; corollary downhills or braking would make it more stable. This seemingly desirable trend is in direct opposition to fork behavior. Similar offsets at the upright made the overall steering a bit quicker at the extremes, thus a bit self-centering; a trait of some value for a big-bore of 53 inch wheelbase and steep rake.
When the supports are non-parallel, another complexity emerges. In this case the two upright links must pivot concentrically with one support, another mechanical impossibity. Otherwise some extra freedom must be introduced into this part of the linkage to prevent suspension movement from straining it. This would hinder motion and likely break components. Hossac allowed one set of pivots to float freely to relieve the strain. In the present case the bellcrank must be divided into two parts mounted concentrically, each now connected independently to the linkages. The errant geometry is then reflected all the way up and becomes a slight "waggle" at the bars as suspension works.
It must be remembered that with nonparallel supports the rake angle varies slightly, here between 26.5.degree. and 27.5.degree.. Thus there is still some bump steer. It is about 0.2.degree. in over ten inches of full travel. The corresponding "waggle" is about 0.1 inch at the grip. Neither would be noticed by a casual rider.
Also, on conventional forks the rider is really twisting them, especially when standing. Because in a linkage system the orientation of the hinges does not depend on the rake angle, they may be placed so the rider more truly pushes and pulls them.
Evidently the major part of the designing such a front end system goes into finding advantages in the necessarily complex steering linkage. Once done, a number of advantages are enjoyed such as: adjustable steering and independent semi-bar progression, elimination of bump steering, compensation for geometry, and lateral reorientation of steering axes to suit rider stance. All of that is achieved in the independent semi-bar concept of this invention.
The U.S. Pat. Nos. 4,388,978 to Frior and 4,179,135 to Slater are herewith incorporated by reference for their showing of suspension systems in motorcycles. For purposes of uniformity the following definitions are set forth. Ther terms "front" and "rear" refer to the normal direction of movement of the cycle. The "longitudinal median plane" of the cycle is defined as the average or middle plane containing the front and rear cycle wheels (this plane being vertical when the cycle moves along a straight line). The "transverse direction" is the direction perpendicular to the longitudinal median plane. A transverse plane is a plane containing a transverse direction (therefore orthogonal to the longitudinal median plane). The "caster angle" is defined in the usual sense as the angle formed between the vertical and the axis around which rotation occurs for steering the vehicle. The "rest position" or "rest attitude" of the cycle is the position of the cycle when it is subjected only to the forces of gravity (at stop or at a constant speed and in a straight line).
The foregoing objects and advantages will become more apparent when taken in conjunction with the following detailed description and drawings showing by way of example a preferred embodiment of this invention.